Liquefaction Hazard Low in Taranaki

Thu 23 May 2013

The possibility of earthquake-caused liquefaction in Taranaki is low, with just a few coastal areas potentially having the type of soil that might liquefy.

However those areas would need to be soil-tested before the potential for liquefaction can be confirmed or discounted.

A report by GNS Science has today (Thursday) been released by the region’s four councils, Powerco and Transpower.

In preparing the report, GNS Science examined existing land data held by South Taranaki, Stratford and New Plymouth district councils and Taranaki Regional Council as well as geological information held by GNS.

The key points in the report are:

• Taranaki’s geology is dominated by materials that are not liquefiable. Potential liquefaction hazards in Taranaki are limited to only a few sites.
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• The potential for liquefaction hazards has been identified at Port Taranaki; and at the lower reaches of the Mohakatino, Rapanui, Tongaporutu, Mimi, Urenui, Onaero and Waitara rivers and their tributaries (in New Plymouth District); and the lower reaches of the Waitotara, Whenuakura and Patea rivers and their tributaries (in South Taranaki).
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• The hazards are based on long time periods: Earthquakes strong enough to cause liquefaction damage to land (a Modified Mercalli Intensity of 8) are expected, on average, every 150 years at Port Taranaki and between 980 and 1070 years at the river areas.
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• If liquefaction does occur, it is expected to be of low impact – nothing like what Christchurch experienced following the 2010 and 2011 quakes.
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“This is not to say that a big earthquake would definitely cause liquefaction at these coastal sites,” says TRC Director – Environment Quality Gary Bedford.

“The report says these sites could have the potential to liquefy when the right conditions are present: Soil type, high water table and a very strong earthquake.

“The only way to know if a hazard does exist is to undertake direct soil testing at these sites.”

New Plymouth District Council will carry out more detailed investigations (cone penetrometer tests) at five river-mouth areas that have urban areas or residential buildings to confirm or discount the liquefaction potential. These areas are at Waitara, Onaero, Urenui, Tongaporutu and Mohakatino.

South Taranaki District Council Environmental Group Manager John McKenzie says STDC will add the GNS report to its hazard information that is used at times of development, such as when a building consent is applied for or when a land information memorandum is produced.

“The risk of liquefaction in South Taranaki is low and the areas identified are not subject to development pressure. However we will follow closely the results of the cone penetrometer tests,” says Mr McKenzie.

NPDC Manager Quality Assurance John Sutton says the study shows that if liquefaction ever did occur, it is likely the effect would be minor.

“This report was initiated as a result of Christchurch’s experience after the earthquakes in 2010 and 2011 and the enormous amount of liquefaction that resulted,” he says.

“The good news from this report is that if liquefaction ever occurs in Taranaki it probably wouldn’t be anywhere near Christchurch’s level of liquefaction, even with an earthquake that produces Modified Mercalli Intensity shaking of 8.”

The New Zealand MM – or Modified Mercalli – intensity scale grades the impact of an earthquake on people, which can be more useful as an indicator of the earthquake’s significance to the community than the absolute size of the earthquake itself. More information on the MM intensity scale is available on GeoNet’s website.

Documents

• Fact sheet (One page 68KB PDF)

• Liquefaction Hazard in the Taranaki Region report (34 pages 8.5MB PDF)

ENDS

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